A spark plug gap tool measures the electrode clearance. It can be a coin-style ramp, a wire gauge, or a feeler gauge. If the gap is too small, the spark will be too weak to burn the fuel. If the gap is too large, the ignition coil won't have enough voltage to jump the gap.
Never press the gap tool against the center electrode pin of precious-metal plugs (like iridium or platinum). The center pin is thin and brittle. You will crack the surrounding ceramic insulator, ruining the plug.